The problem of fluid loss to highly porous underground formations penetrated by a well has, of course, been long recognized. These highly porous zones are often called thief zones. In water or steam stimulation operations, for example, a serious problem is often encountered because a very small interval of the total production zone may be taking 80 percent or more of the total injected fluid. When this happens, the benefit of the injection project may be lost or greatly reduced.
An isolated high-permeability zone or fracture can be plugged at the well bore face by a shallow layer of applied cement, though such a permanent relatively irrevocable technique often is undesirable. More desirably, a communicating high-permeability zone is plugged to some considerable depth in order to prevent flood water from otherwise merely flowing around a narrow shallow plug and back into the high-permeability or swept zone. Indepth plugging of a relatively high-permeability zone converts the zone into a much lower permeability zone. Then subsequently injecting flood water or other fluid will tend to enter the formerly by-passed but now relatively more permeable hydrocarbon-bearing zones and thus mobilize increased amounts of hydrocarbons therefrom.
Various methods have been used in the past to achieve indepth gelling, such as gelable systems triggered by a following aqueous acidic solution injection for subsequent pH adjustment. However, injecting an acidic solution following the polymer solution may result in gelatin occurring so rapidly that a sufficient indepth plugging is not effectively obtained in the most permeable strata where desired. In another method, water, a polymer and a crosslinking agent capable of gelling the polymer such as a sequestered polyvalent metal cation, are admixed, and, just before injection into an underground formation, an acid is added thereto to effect gelation. But, when the acid is pre-mixed with the gelable composition, the gelation can be too fast, making it necessary to shear the gelled polymer in order to be able to obtain adequate injection, which reduces effectiveness of the gel.
Indepth gelling has also been effected by the controlled gelation of sodium silicate. Also polymers have previously been gelled in permeable zones by borate ions supplied in various ways.
According to this invention the permeability of highly permeable zones in a subterranean formation is reduced by introducing to the formation an aqueous solution of a water soluble polymer which is crosslinked to form a gel by borate ions and an alkoxy amine ester of boric acid which hydrolyzes sufficiently slowly to allow the polymer and ester to penetrate the formation before boric acid is released and converted to borate ions which crosslink and gel the polymer.